Gain control circuit



Nov. 8, 1938.

J. "r. DIXON A 2,135,566

GAIN CONTROL CIRCUIT I Filed Sept. 2, 1937 2 Sheets-Sheet l FIG! anuaw-METER colvmoz -20 i :d O

INVENTOR 6y. JJTD/XON ATTORNEY Nov. 8, 1938.

\ J. T. DIXON 1 2,135,566

GAIN CONTROL CIRCUIT Filed Sept. 2, 1957 2 Sheenbs-Sheet 2 GAL v4 NOMETER MECHANISM FIG. 6 /9 GALIMIVOMETER MECHANISM INVENTOR J. 7: D/X ONGILVANOMETER W MECHANISM ATTORNEY Patented Nov. 8, 1938 PAT- ENT orricaGAIN CONTROL CIRCUIT John T. Dixon, New York, N. Y., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application September 2, 11937,- Serial No. 162,137

Claims.

line having sections varying differently with temperature changes thatshall automatically compensate for the different changes in attenuationin'the transmission line sections to maintain the line. attenuationconstant. 7

Another objectof theinvention is to provide pilot wire gain controlcircuits for a transmission linehaving sections with differentattenuation-temperature coefficients that shall control the transmissionline attenuation while compensating for the difier'ence in theattenuation-tem- Fahrenheit temperature change.

perature coefficients of I the transmission line sections in an improvedmanner to maintain the transmission line attenuation constant.

A further object of the invention is to provide pilot wire gain controlcircuits for a transmission line having sections with differentattenuationtemperature coefficients that shall adjust the transmissionline attenuation according to the resistance changes of a pilot wirecircuit associated with all the line sections and according toresistance changes of a pilot wire circuit associated with one linesection, while compensating the attenuation controlling operationaccording to the diiference between the attenuation-temperaturecoefficients of the line sections.

The variations in attenuation in many types of conductors fortransmitting signals are generally caused-by,- changes in temperature..thermore, the changes in the attenuation in the conductors varydirectly with the changes in tem- Furperature. A pair of wires in acable is an exampleof the signal conductors under consideration. Inalong cable line a portion of the cable maybe located above ground and aportion thereof maybe located below ground. The different differentsections of 'a pair of conductors of a cable dilficulty is encounteredin maintaining the attenuation of the pair of conductors constant. The'term attenuation-temperature coefficient which is employed in explainingthe invention may be defined as the decibel attenuation change per mileof a transmission line per degree The term attenuation-resistancecoefficient of a-conductor system may be defined as the decibelattenuation change per ohm change in resistance.

of a conductor are definitely related, a definite relation existsbetween the attenuation-temperature and"attenuation-resistancecoefficients for a line section of given length. Because of the aboverelation it has been customary in the'past to use the resistance changesin a direct-current pilot wire circuit'forcontrolling a gain adjustmentcircuit to effect compensation in a transmission line attenuation inproportion to the resistance changes in the pilot wire circuit.

A single pilot wire control circuit operates very satisfactorily forcontrolling the attenuation of a transmission line if the transmissionline is not madeup of sections having different attenuation-temperaturecoefficients. If the sections of a transmission line do not have thesame attenuation-temperature coefiicients errors in compensating forchanges in attenuation because of temperature changes will take place.The errors in compensation may in some case become serious in magnitude.In the abovementioned example of a cable having a section undergroundand a section above ground that change attenuation with temperaturechanges at different rates, some compensation must be efiected for thediiference between the attenuation-temperature coefficients of thedifferent line sections.v

The invention provides means in a pilot wire gain control circuit for atransmission line having sections with difierentattenuation-temperaturecoefiicientsto compensate for the difference between theattenuation-temperature coefficients of the different sections andmaintain the transmission line attenuation constant. In a gain controlcircuit constructed in accordance with one form of the invention, apilot wire circuit which is associated with two different sections of atransmission line is included in one arm of a bridge circuit. The bridgecircuit may operate galvanometer control mechanism of the type disclosedin the patent to Ira G. Wilson, No. 2,086,105, July 6, 1937, or in thepatent to J. A. Coy et al. 2,017,654, dated Gctober 15, 1935, to controlthe attenuation of the transmission line.

The different sections of the transmission line with the differentsections of the pilot wire circuit will have difierentattenuation-resistance coefiicients.

The bridge and the galvanometer control mechanism may be set to controlthe transmission line attenuation according to the resistance variationsof the pilot Wire circuit associated with all sections of thetransmission line and the at-.

tenuation-resistance coefficient of one line section. Although thetransmission line attenuation changes and the pilot wire resistancechanges are caused by the same temperature changes, the transmissionline attenuation will not be held constant because the attenuationcontrolling apparatus is only set according to the attenuationresistancecoefiicient of one line section. An error will occur in the adjustmentof the line attenuation according to the difference between theattenuation-resistance coefiicient of the sections and the resistance ofthe associated pilot wire a section.

One way to correct the above-mentioned error in the adjustment of thetransmission line attenuation is to provide a second pi'lot'wire circuitassociated with only one section of the transmission line. Resistanceelements are connected in series and across the second pilot wirecircuit which is included in an arm of the bridge circuit to the bridgearm including the first-mentioned pilot wire circuit. The second pilotwire .circuit with the resistance elements serve to adjust the bridgeand compensate for the difierence between the two attenuation-resistancecoeificients.

In the accompanying drawings:

Fig. 1 is a diagrammatic'view of a transmission line having sectionsthereof 'changingiattenuation at diiierent rates with temperaturechanges;

, Fig. 2 is a diagrammatic view of a control circuit for maintaining theattenuation constant on the transmission line shown 'inFig. 1;

Fig. 3 is a diagrammaticview of a'modification of the controlcircuitsshown .in'Fig. 2;

Fig. 4 is a diagrammaticview illustrating apparatus for governing thecontrol circuit shown in Fig. 2;

Fig. 5 is a diagrammaticviewsillustrating apparatus for governing thecontrol circuit shown in Fig. 3; v

Fig. 6 is a diagrammatic view illustrating apparatus for governing thecontrol circuit shown in Fig. i.

Referring to Fig. 1 of the drawings, a transmission line comprisingconductors 1 and 2 may be a pair of wires in a cable. The transmissionline is assumed to be divided into sections 3 and '4 which are subjected'to different attenuation varying conditions. One of the line sectionsmay be located below ground and the other section may be located aboveground. .Apilot wire circuit 8 which forms onearm of a bridge circuit 9has two sections 3 and4' respectively associated with the line sections.3 :and 4., and subjected'to the same temperature 'varying :conditions.The bridge circuit .9 serves to control the attenuation of thetransmission line comprising :conductors I and 2 as will be describedlater. The bridge has three arms formed by resistance elements .l5, l6and ii and an adjustable resistance I 3.

The resistance element 13 is adjusted to rebalance the bridge afteroperation thereof by reason of changing of theresistan'ce in the pilotline 8. A source of potential I8 is connected'across two vertices of thebridge. A galvanometer 21] serves to control the rebalancing of thebridge and to control the attenuation of the transmission line.

A pilot wire gain control circuit of the type shown in Fig. 1 of thedrawings will effectan error in adjusting the transmission lineattenuation which may be determined. Let in be theattenuation-resistance coefiicient of section 4 in the cable and 002 bethe :attenuatiomresistance .coefficient of section 3 in the cable. Letthe change in resistance from a reference condition, due to temperaturechanges be AR1 in section 4 and A132 in section 3. According to thedefinitions above set forth the changes in attenuation of the twosections 3 and 4 in series will be The apparatus controlled by thebridge 9 and the galvanometer 20 may be adjusted to compensate the lineattenuation at any desired .rate per ohm change in the resistance of the:pilot circuit 8. Let the compensation be introduced at the rate of a3decibels per ohm change in the pilot wire circuit 8. The magnitude ofthe gain correction which will be introduced by the gain control circuitwill .then be given by the expression as(AR1+AR2). Subtracting thechange in attenuation of the cable from the gain correction gives theexpression as the error in the compensation. It is apparent from theform of this expression that an error arises because the quantities a1and m are different and therefore cannot be' made simultaneously equalto the quantity us. Some simplification results without loss ofgenerality if an is assumed to be greater than a1 and-if as is madeequal to (1 2. The error in compensationthen becomes According to theinvention the operation of the gain control circuit is controlled toprevent the above error. This is 'done' by an arrangement whichautomatically causesithecontrol mechanism to correct for a resistancechange of ARi-I-ARz-IL instead of AR1+ARz. The quantity a: is determinedso as to give zero error. The

a2 AR;

One method for automatically introducing a resistance correction of thedesired value in'the control circuits, in accordance with thisinvention, is shown by Fig. 2 of the drawings. In Fig. 2 of the drawingslike parts to those shown in Fig. 1 have been indicated by similarreference characters. A second pilot wire circuit I I) is placed in asecond arm of the bridge 9 adjacent to the arm containing the firstpilot wire circuit 8. The second pilot wire circuit l0 extends only thelength of cable section 3. Associated with the pilot wire circuit ID aretwo resistances R1 and R2, R1 being in series and R2 in shunt withrespect to the resistance of the pilot wire circuit. The value of theresistances R1 and R2 are so proportioned to the resistance .R0 of thepilot wire circuit ID that a change in resistance of ARo in the pilotwire circuit appearsas a change of only ARQ to the bridge 9. The pilotwire circuit I0 and its associated resistances, being in an adjacent armof the bridge, effectively subtract from the total resistance changeAR1+AR2 of pilotcirc-uit 8 and therefore introduced the deisiredcorrection as expressed by Equation '(2) above.

If the resistance-temperature coeflicient a: were taken as less than (M,then the desired correction could be obtained by connecting thecombination of resistances R1, R2, and pilot wire III in series withpilot wire circuit 8, in the same arm of the bridge. I

The proportioning of the resistances R1 and R2 with respect to theresistance R of pilot wire circuit ID will be made clear with referenceto the/equivalent circuit shown in Fig. 3 of the drawings.

The equivalent resistance of this embodiment. is

and the equivalent resistance of the combination when R0 is changed toRo-I-ARo is R.+'RO+ARO)RZ (4) 1+ o+ 0+ 2 V It is readily shown bysubtraction of Equation (3) from Equation (4) that the change inresistance of the combination is AR R22 1 (5) 1+ o+ 2) o l+ O+ 2 Thequantity will not differ greatly from unity since the ratio is of theorder of .12 for aerial conductors.

The resistance R1 serves to improve this approach to unity and is chosenas high as is consistent with practical considerations involving theoperation of the bridge with its control mechanism. Approximately thequantity may be taken to be unity and the resistance change of thecombination of Fig. 3 becomes equal to R22 a a The quantities a1, a2 andR0 are determined by measurment on the cables, the quantity R1 is vchosen as stated in the immediately preceding paragraph, and thequantity R2 is chosen to fulfill Equation (6).

. A second method for making the desired resistance correction inaccordance with Equation (2) will be explained with reference to Fig. 4of the drawings. In Fig. 4 of the drawings like parts to those shown inFig. 2 of the drawings will be indicated by similar referencecharacters. In Fig. 4 the gain control mechanism is controlled by theunbalance of the bridge 9. An auxiliary bridge 48 has a pilot wirecircuit ID in one arm and this pilot wire circuit extends only cent tothat containing pilot wire circuit 8 but it is to be understood thatconnection in any one of the arms is contemplated as required. Theresistance of the element 52 is so chosen that a resistance change ofARz in pilot wire circuit l0 will'cause a resistance of to be introducedin bridge 9, thereby satisfying the condition for zero error as statedby Equation (2). The resistance elements 44 and 52 are controlledsimultaneously by means controlled by the galvanometer 48.

The preceding exposition has referred to a line section made up of onlytwo cable sections of difierent type with only two pilot wires used toobtain improved compensation for changes in line attenuation. It is tobe understood that the means herein described are not restricted to thisparticular condition but are applicable to more than two line sections.In such cases, a plurality of pilot wire circuits are required and thesemay be associated with resistance networks and a single bridge as hasbeen described with reference to Fig. 1 or they may be associated with aplurality of bridges as has been described with reference to Fi .4.

Referring to Fig. 5 of the drawings apparatus is shown for controllingthe attenuation in the line comprising conductors I and 2 according tothe operation of the bridge shown in Fig. 2 of the drawings. Like partsin Fig. 5 of the drawings to those shown in Fig. 2 of the drawings areindicated by similar reference characters.

The transmission line comprising conductors l and 2 may be a pair ofwires in a cable. The transmission line may be divided into sections 3and 4, which are subjected to different attenuation varying conditions.For example, one of the sections may be located below ground and theother section may be located above ground. An amplifier 5 of anysuitable type may be provided for controlling the net attenuation of thetransmission line. A potentiometer 6 having a control arm I may beconnected to the conductors l and 2 ahead of the amplifier 5 forcontrolling the gain of the amplifier. The pilot wire circuit 8 which isassociated with sections 3 and 4 of the transmission line is included inone arm of the bridge circuit 9. The second pilot wire circuit H), whichis associated only with section 3 of the transmission line, is shown inanother arm of the bridge 9 adjacent to the bridge arm including thepilot line 8. The pilot wire circuit I0 is provided with a seriesresistance R1 and a resistance Rz connected across it for compensatingfor the difference in attenuation varying conditions of the difierentline sections, as will be set forth hereinafter.

The adjustable resistance I3 in the bridge circuit 9 is under control ofan arm M for rebalancing the bridge. The galvanometer control mechanismcomprises the galvanometer 20 which governs the operation ofgalvanometer mechanism 25 according to the unbalance of the bridgecircuit 9. The galvanometer mechanism 2| may be of the type disclosed inthe above-mentioned patent to I. G. Wilson, No. 2,086,105 or in thepatent to J, A. Coy et al., 2,017,654. The galvanometer mechanism 2|selectively operates two switches 22 and 23, which respectively controltwo relays 24 and 25. The relay 24 is. provided with two switch members25 and 27 and the relay 25 is provided with two switch members 28 and29.

:Theswitch members under control of the relays 1-24 and 25 connectasource'30 to a motor 3| for operating it in a forward or a reversedirection. The motor 3! as shown in the drawings isexcited by means of'abattery .32. A pinion 33 mounted on the .armature shaft of the motor 3|meshes with a gear wheel 34 on a shaft 35. The shaft 35 controls thepotentiometer arm I for governing the operation of the potentiometer 6.

The sections 3 and 4 of the transmission line will have differentattenuation-temperature coefiicients. The term, attenuation-temperaturecoeificient, as before set forth mat be considered as the decibelsattenuation change permtile per degree Fahrenheittemperature change.Furthermore, the attenuation-resistance coefficients for the linesections 3 and 4 will not be the same. The term, attenuation-resistancecoefficient, as before set forth may be considered as the decibelsattenuation change in the transmission line per ohm resistance change inthe pilot line. A change in the temperature will cause a change in attothe attenuation variation .of the transmission line, because of thedifference between the attenuation-resistance coefficients of the twoline sections. The bridge 9 and the galvanometer controlmechanism l9when controlled solely by the pilot wire circuit 8 can onlybe operatedin accordance with the attenuation-resistance coefficient of one linesection. If the bridge 9 and the galvanometer control mechanism H! areoperated solely according to the resistance of the pilot wire circuit 8and the attenuation-resistance coeificient of the line section 4, thenan error in the net attenuation of the transmission line will beeffected.

The second pilot line [0 will effect a correction equal to where m andas are, respectively, the attenuationresistance coefficients of the linesections 3 and 4, and AR1 is the resistance variation of the pilot wirecircuit l9 associated with the line section 3.

The bridge circuit 9, when unbalanced by a variation of resistance ofthe pilot wire circuit, operates the galvanometer 20. The galvanometer2G in turn operates the galvanometer mechanism 25 to close either theswitch 22 or the switch 23, according to whether there is an increase ora decrease in the resistance of the pilot wire. The galvanometermechanism 2| also operates a shaft to for controlling the arm I4associated with the resistance 13 to rebalance the bridge circuit. The

switch 22, if operated, completes a circuit from battery 31 foroperating the relay 24. The switch 23, if operated, completes a circuitfrom battery 33 for operating the relay 25. The relays 24 and 25 operatethe motor 3i in a forward or in a reverse direction to control thepotentiometer 6 and accordingly the attenuation on the transmissionline.

Referring to Fig. 6 of the drawings, apparatus is shown to compensatefor the difference between the attenuation-resistance coefficients ofthe different line sections in a different manner.

Iiikepartsin the circuit-shown in Fig. 6-to those shown in Fig. 4 willbe indicated by similar refer-' ence characters.

with the adjusting resistance elements R1 and 1R2. and is :not connecteddirectly to the bridge 92s in the circuit shown in Fig. 5 of thedrawings. The pilot wire circuit I9 is. included in'an arm of theauxiliary bridge circuit 40. The :bridge 40 comprises resistance arms4|, 42, 43 and :a resistance 44 under control of a switch arm '45. Abattery 46 is connected across two opposite vertices of the bridge 40.Galvanometer control mechanism 47 whichis similar to the galvanometercontrol mechanism l9 referred'to in Fig. 5 of the drawings is controlledby the bridge 40. The galvanometer control mechanism 41 comprises agalvanoniet'er 48 which operates galvanometer mechanism 49. Thegalvanometer mechanism 49 operates a shaft 50, which not only controlsthe switch arm 45 for adjusting the resistance 44 .to rebalance thebridge 49 after each] operation thereof, but also operates an arm 5|which controls aresistance 52. The resistance 52 is included in an armof bridge 9. The resistance 52 is shown connected in an arm adjacent tothe arm. including pilot wire 8 but resistance 52 may also be connectedin the same arm as pilot wire circuit 8. The resistance 52 controlled bythe galvanometer. control mechanism 4'! serves to compensate theoperation of the bridge 9 according to the resistance of the pilot wirecircuit 8 associated with the line section andaccoriding to.thedifference between the attenuation-resistance coefiicients of thetwo line sections 3 and 4. The proper compensation depends on the properselection of the resistance value of' resistance 52 as before set forth.The bridge 9 and thega'lvanometer control mechanism i9 operate in thesame manner as that described with respect to the circuit shown in Fig.1 for controlling the potentiometer 6 to maintain the line attenuationconstant.

Modifications in the" circuits and in the arrangement of parts may bemade within the...

spirit and scope of the invention and such modifications are intended tobe covered by the appended claims.

What is claimed is:

l. A transmission line divided into sections having differentattenuation-resistance coeiiicients, a pilot Wire circuit associatedwith all of said line sections, control means adjusted according to theattenuation-resistance coefficient of one line section-"and according tothe resistance variations of the pilot wire circuit to compensate forattenuation changesof the transmission line, a second pilot wire circuitassociated with only one line section and 'means for adjusting saidcontrol means according to the re- 4 means comprising a second pilotwire circuit as-' The second pilot wire circuit I I0 shown in Fig. '6 ofthe drawings is not provided sociated with only one line section foradjusting said control means and resistance elements in said secondpilot Wire for reducing the control effected by the second pilot wire soas to correct for an error effected in compensating for the change intransmission line attenuation by the first-mentioned pilot wire inaccordance with the attenuation-resistance coefficient of only one linesection.

3. Atransmission line divided into sections having differentattenuation-resistance coefficients, a pilot wire circuit associatedwith all of said line sections, a bridge operated according to theattenuation-resistance coefficient of one line section and according tothe resistance variations of the pilot wire circuit to compensate forattenuation changes of the transmission line, a second pilot wirecircuit associated with one line section and connected to an arm, ofsaid bridge and means for reducing the control of the bridge by thesecond pilot wire circuit to compensate for the error effected inadjusting the transmission line attenuation according to theattenuation-resistance coefficient of only one line section.

4. A transmission line divided into two sections having differentattenuation-resistance coefiicients, a pilot Wire circuit associatedwith both of said line sections, a bridge having said pilot wire circuitincluded in one arm thereof, a second pilot wire circuit associated withone line section and included in an arm in said bridge adjacent to thearm including said first mentioned pilot wire circuit, means forreducing the effect of said second pilot wire circuit to adjust saidbridge according to the difference between the attenuation-resistancecoefficients of the two line sections, and means controlled by saidbridge for maintaining the attenuation of the transmission lineconstant.

5. A transmission line divided into sections having differentattenuation-temperature coefficients, a pilot wire circuit associatedwith all of said line sections, a bridge circuit having said pilot wirecircuit included in one arm thereof, means operated by said bridgeaccording to the attenuation-resistance coefficient of one line sectionor governing the transmission line attenuation according to theresistance variations of the pilot wirecircuit', and means forcontrolling an arm of said bridge adjacent to the arm including saidpilot wire according to the resistance variations in one line sectionand the difference between the attenuation temperature coemcients tocorrect for the error effected in adjusting the transmission lineattenuation in accordance with the attenuation-resistance coefficient ofonly one line section.

6. A transmission line divided into two sections having differentattenuation-resistance coefficients, a pilot wire circuit associatedwith both of said line sections, a bridge having said pilot wire circuitincluded in one arm thereof, means operated by said bridge for adjustingthe transmission line attenuation according to the resistance variationsof the pilot wire circuit and the attenuation-resistance coeflicient ofone line section, a second pilot wire circuit associated with theattenuation-resistance coefficients of the two line sections.

7. A transmission line divided into two sections having difierentattenuation-resistance coefiicients, a pilot wire circuit associatedwith both of said line sections, a bridge having saidpilot wire circuitincluded in one arm thereof, a second pilot wire circuit included in anarm of said bridge adjacent to the arm including said first mentionedpilot line and associated with i only one line section, said secondpilot wire circuit having resistance elements connected in series and inshunt for adjusting said bridge according to the difference between theattenuation-resistance coefiicients of the two line sections, and meanscontrolled by said bridge for maintaining the attenuation of thetransmission line constant.

8. A transmission line divided into sections having differentattenuation-resistance coefiicients, a pilot wire circuit associatedwith all of said line sections, a bridge circuit having said pilot wirecircuit included in one arm thereof, means controlled by said bridge forgoverning the transmission line attenuation according to the resistancevariations of the pilot wire circuit and according to theattenuation-resistance coefiicient of one section of said transmissionline, a second pilot line associated with one section of saidtransmission line and included in an arm of said bridge, and impedancemeans included in said second pilot wire circuit for correcting thebridge balance to compensate for the error effected in adjusting thetransmission line attenuation according to the attenuation-resistancecoeificient of only one line section.

9. A transmission line divided into two sections having differentattenuation-resistance coefficients, a pilot wire circuit associatedwith both of said line sections, a bridge having said pilot wire circuitincluded in one arm thereof, means operated by said bridge for adjustingthe transmission line attenuation according to the resistance variationsof the pilot wire circuit and according to the attenuation-resistancecoefficient of one line section, a second bridge circuit, a second pilotwire circuit associated with only one line section and included in anarm of said second bridge circuit, and means operated by said secondbridge for adjusting said first men- I,

tioned bridge according to the difierence between theattenuation-resistance coefficients of the two line sections to maintainthe attenuation of the transmission line constant.

10. A transmission line divided into two sections having differentattenuation-resistance coefficients, a pilot line circuit associatedwith both of said line sections, a bridge having said pilot wire circuitincluded in one arm thereof, means operated by said bridge for adjustingthe transmission line attenuation according to the resistance variationsof the pilot line and according to the attenuation-resistancecoefficient of one line section, a second bridge circuit, a second pilotwire associated with only one line section and included in an arm ofsaid second bridge, and means operated by said second bridge foradjusting an arm of said first mentioned bridge to compensate for theerror efiected in adjusting the transmission line attenuation accordingto the attenuation-resistance coefficient of only one line section.

JOHN T. DIXON.

